Holden is making another of his close observations about other people; how they act, how they behave, how they dress. In this case, he's referring to Mrs. Morrow's penchant for wearing lots of expensive jewelry. In this specific context, "lousy" doesn't mean "very bad" as in "this coffee's lousy" for example. Instead, it suggests an infestation of lice. Only Mrs. Morrow isn't infested with any kind of bugs or critters, but precious gemstones, or "rocks."
Though super-judgmental as always, it seems that Holden kind of likes Mrs. Morrow, which is especially unusual given that her son is someone Holden regards as a "bastard." Indeed, the very thing that seems to have drawn Holden to Mrs. Morrow is his sense that she understands precisely what kind of a "bastard" her son really is. But then Holden immediately reverts to type and distances himself from Mrs. Morrow, insisting that mothers are all slightly insane.